X-ray tubes are for example used in CT systems wherein the X-ray tube is rotating about a patient, generating a fan-beam of X-rays, wherein opposite to the X-ray tube and with it on a gantry rotor rotates a detector system which converts the attenuated X-rays into electrical signals. Based on these electrical signals, a computer system may reconstruct an image of the patient's body.
In the X-ray tube, a beam of primary electrons emitted from a cathode hits a focal spot of an anode and creates X-rays. However, a percentage of the incoming primary electrons is backscattered or creates recoil electrons, these electrons being hereinafter commonly referred to as back-directed electrons. Thereby, it is converted to a current of back-directed electrons leaving the focal spot and carrying approximately 40% (W-target) of the energy of the primary beam away.
Some conventional tube designs have the cathode directly in front of the anode. Accordingly, a strong electrical field is established between the negative cathode and the positive anode. In such tube designs, due to mirror effects caused by the positively charged anode, a lot of back-directed electrons are redirected again to the anode which is thereby heated in an undesired way and which furthermore creates undesired off-focal radiation from areas spaced apart from the focal spot where the back-directed electrons impact onto the anode.
In an improved recent approach, such undesired heating and off-focal radiation can be effectively reduced by improving the X-ray tube in such a way that the back-directed electrons are allowed to travel in a nearly field-free space towards collector electrodes. So, most of the 40% of the heat loading of the anode may be avoided and also most of the off-focal radiation may be avoided.
However, there may arise problems due to ions created from residual gas or target vapour in the intense primary beam and in the shower of back-directed electrons. In earlier X-ray tube designs in which a strong electrical field is created between the anode and the cathode, such ions are attracted by the electrical field towards one of the electrodes, usually the cathode. However, in recent X-ray tube designs having nearly field-free spaces, the ions may no longer experience a strong electric pull field. Accordingly, there is no relevant ion pump active anymore, as in former designs, where large amounts of ions were implanted into the cathode and removed from the vacuum within the X-ray tube.
Accordingly, X-ray tubes with a long nearly field-less drift path of the useful electron beam, where the electrostatic field in the vicinity of a major part of the useful electron beam for the generation of X-rays is smaller than the dynamic field generated by the beam space charge, may suffer from substantial ion concentrations in the electron beam which may de-stabilize its focusing.